For use in a duct system installed in a vulcanizing plant or a record disc press machine through which hot steam passes, the considerations of saftey and operational reliability for a valve forbids the use of electrical valves which can be automatically opened and closed. A diaphragm or a piston operated valve which utilizes high pressure air operated actuating means is used in the duct system for fluid such as hot steam.
However, although a diaphragm operated valve is constructed to open or close the valve by driving a piston through a diaphragm which is formed of a flexible material such as synthetic rubber, the material of the diaphragm itself does not have a sufficient strength and durability, and this causes a damage thereof as a result of a degradation in the quality of the material over a prolonged period of use or the influence of mixtures contained in the high pressure air which is used to operate the valve. Any damage to the diaphragm may rapidly change the flow path, which disadvantageously may cause a serious accident due to the blowout of steam in a duct steam which utilizes the hot steam.
In the conventional construction, a diaphragm operated valve is used under the condition that the diaphragm and the piston are disposed in abutment against each other whenever the high pressure air is applied. Accordingly, there is a need to provide an operating clearance on the part of the diaphragm, resulting in a construction that the piston has a reduced surface area which is subject to the pressure while the diaphragm has an increased surface area which is subject to the pressure. As a consequence, the air pressure which is used to operate the valve is split into two portions, one acting upon the piston and the other upon the peripheral region of the diaphragm. This leads to the difficulty that a valve loading which has a great influence upon the isolating and sealing characteristic of the valve cannot be simply determined by a calculation based on the air pressure and the surface area of the piston which is subject to the pressure.
To avoid these disadvantages of the diaphragm operated valve concerning the safety and the isolation/sealing characteristic which occur when it is used in a hot steam duct system, there has been a proposal to substitute a piston operated valve for the diaphragm operated valve. Since it does not include any structure such as a diaphragm which may be damaged, a piston operated valve can assure a high safety. In addition, since only the piston is subject to the high air pressure, the valve loading can be precalculated to enable a high degree of isolation/sealing characteristic to be assured, thus making such valve preferred for use in a duct system which utilizes hot steam or the like.
However, a piston operated valve found in the prior art comprises a valve body which is formed of a metal material. Hence, a metal seal must be used which is known to present a difficulty in forming suitable sealing surfaces, resulting in a degraded sealing performance. In addition, the valve body must be frequently changed because of its abrasion.
To provide an improvement in the conventional piston operated valves, Japanese Laid-Open Utility Model Application No. 55-105666 proposes a piston operated valve in which a valve body is formed of a fluorocarbon resin rather than metal. As is recognized, a fluorocarbon resin has a degree of elasticity, and permits a seal having an increased area of contact to be constructed, thus allowing a high sealing performance to be achieved. However, a reduced abrasion resistance represents a disadvantage of such valve, which cannot be used over a prolonged period of time. The reduced life of the valve body requires an increased maintenance labor.
Accordingly, it is an object of the invention to provide a piston operated valve having improved sealing characteristic and durability while eliminating the disadvantages of conventional piston operated valves.
It is another object of the invention to provide a piston operated valve having an improved heat and pressure resistance and which permits a facilitated replacement of valve body.